Method of manufacturing a multi-layered ceramic capacitor

ABSTRACT

A low-temperature sinterable dielectric composition with a high dielectric constant wherein x=2-4 mol %, y=0.5-3.0 mol % and z=3-5 mol % in [95PMN-5PT]-x(PbO)-y(CaO)-z(CuO).

This is a division of application Ser. No. 08/577,973, filed Dec. 22, 1995 now issued U.S. Pat. No. 5,680,291.

BACKGROUND OF THE INVENTION

The present invention relates to a low temperature sinterable dielectric composition with a high dielectric constant, a multi-layered ceramic capacitor using the same, and a manufacturing method therefor, and more particularly, to a dielectric composition having a high dielectric constant, anti-reduction characteristics and low-temperature sinterability, a multi-layered ceramic capacitor having a copper internal electrode using the composition, and a manufacturing method therefor.

Recently, along with the high-density integration of electronic components and circuits of high frequencies, capacitors have been required to have larger capacitance and be more miniature in size. This increases the demand for multi-layered ceramic capacitors (hereinafter refer to as MLCs). The miniaturized size and higher density of MLC requires its dielectric film to be thinner and more stacked, and to have a higher dielectric constant. Accordingly, the biasing and signal voltage characteristics of the dielectric material must be improved against the deterioration of dielectric characteristics caused by the usage of a low-cost internal electrode and thin-film construction.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to reduce the fabrication cost of a high-capacitance multi-layered MLC and to enhance its dielectric characteristics using a low-cost internal electrode and a dielectric ceramic composition of high dielectric constant.

In order to accomplish the object of the present invention, there is provided a low-temperature dielectric composition with a high dielectric constant and anti-reduction characteristics, and a method of fabricating an MLC with a copper internal electrode using the composition.

BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS

FIG. 1 is a cross-sectional view of a multi-layered ceramic capacitor of the present invention;

FIG. 2 shows the temperature profiles during an organic binder burn-out and presintering of the present invention;

FIG. 3 shows an illustration of specimens loading during sintering of the present invention;

FIG. 4 is a block diagram of a reduction atmosphere firing system of the present invention; and

FIG. 5 shows firing temperatures profile of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, preferred embodiments of the present invention will be described below with reference to the attached drawings, but the present invention is not limited to these.

Generally, in order to use a base metal such as Ni or Cu as an MLC internal electrode, such base metal must be sintered under low oxygen partial pressure, that is, a reduction atmosphere to prevent oxidation during firing. Cu has a melting point of 1,083° C. thus its sintering temperature must be less than the point by at least 50° C. in case Cu is to be used for internal electrode production. When copper is used for the internal electrode, the dielectric ceramic is in a reduction atmosphere where oxygen partial pressure is 10⁻³ Pa at 900° C. The reason why copper is selected as the material of the internal electrode is because it has a higher equilibrium oxygen partial pressure than that of base metals such as Ni, Fe and Co, and thus there is less reduction effect to the dielectric material.

For the limit oxygen partial pressure at which the dielectric material is reduced, a equilibrium oxygen partial pressure where, of elements forming the dielectric material, an element easiest to be reduced has a lower valence from a higher valence, should be considered. For an MLC using copper as its internal electrode and in consideration of the dielectric's anti-reduction characteristics, the dielectric material must not be reduced until an oxygen partial pressure which is lower than copper's equilibrium oxygen partial pressure, is reached below the sintering temperature, and must also exhibit high resistivity.

J. Kato reported that in case of a PMN-PT-PNW composition, excessive A-site or adding alkaline-earth metal results in anti-reduction characteristics. For excessive A-site, Pb is added excessively or Pb is replaced with Ca. Here, it is known that, Ca may be replaced by Ba or Sr, to achieve the same effects.

In the present invention, excess A-site and alkaline-earth metal addition are both employed in order to guarantee that the Pb-based dielectric composition has anti-reduction characteristics. Specifically, as a reduction-resistant agent, PbO of over 2 mol % and CaO of 0.5-3.0 mol % are added to the main PMN-PT composition for its reduction-resistance. Further, as the sintering aid agent and inhibitor of pyrochlore phase formation, CuO is added to form a liquid phase due to the eutectic point of PbO--CaO--CuO, for lowering the sintering temperature and suppressing the formation of pyrochlore phase to enhance the dielectric constant. A method of manufacturing the dielectric ceramic composition of the present invention will be described below.

As starting materials, reagents of PbO, MgO, Nb₂ O₅, TiO₂ and CuO are used. For CaO when added in minute amounts, CaCO₃ is used for the purpose of uniform mixing.

First, after the starting materials are made into a powder and dried for 12 hours at 120° C. and weighed, it is wet-mixed for one hour in a centrifugal planetary mill with a zirconia ball and jar. MgO and Nb₂ O₅ are mixed and calcined at 1,100-1,200° C. to form a columbite precursor (MgNb₂ O₆). This precursor is reacted with PbO at 900° C. for two hours, thereby forming Pb(Mg_(1/3) Nb_(2/3))O₃ (hereinafter, refer to as PMN). PbTiO₃ is made by mixing PbO and TiO₂ by 1:1 ratio and calcining this mixture at 800° C. for two hours. In the present invention, the heating rate of firing is 300° C./h.

The two main compositions described above are mixed at a ratio of PMN:PT=95:5 and dried while PbO of 2-4 mol%, CaO of 0.5-3 mol % and CuO of 3-5 mol % are added thereto. The resultant is calcined for two hours at 750-850° C. The calcined powder is then pulverized and dried for two hours. Then, PVA binder of 2 wt % is added thereto, and disk specimens of 10 mm in diameter and 1.2mm in thickness are formed. These specimens are sintered at 810-1,100° C. in air and then sintered for 1 hour at 800° C., oxygen partial pressure P₀₂ of 10⁻¹¹ atm, in a reduction atmosphere as a cooling process. Ag paste is coated on both sides of the specimens, and then fired for ten minutes at 600° C. under N₂ atmosphere, to form electrodes.

The completed specimens are measured using an LCR meter and high resistance meter after charged for one minute, under the conditions of a capacitance of 1 KHz, a dissipation factor of 1 V_(rms) at normal temperature (20° C.), and an insulation resistance of 50 V_(DC). The following table 1 shows examples of those compositions and their dielectric properties.

                                      TABLE 1                                      __________________________________________________________________________     Dielectric Properties of [95PMN-5PT]-x(PBO)-y(CaO)-z(CuO) composition                                 Dielectric Properties                                                               Dielectric                                                                          Insulation                                    Additives (mol %)                                                                             Sintering                                                                              Dielectric                                                                          Loss resistance                                    x(PbO) y(CaO)                                                                             z(CuO)                                                                             temperature (° C.)                                                              constant                                                                            (%)  (Ω · cm)                       __________________________________________________________________________      1 0   0   0   1100    16500                                                                               2.4  9 × 10.sup.3                             2 2.0 0.5 0   1000    13100                                                                               1.4  8 × 10.sup.10                           *3 2.0 0.5 3.0 900     14700                                                                               1.5  1 × 10.sup.11                           *4 2.0 0.5 5.0 900     14200                                                                               1.8  9 × 10.sup.10                            5 2.0 1.0 0   950     11800                                                                               1.0  4 × 10.sup.11                           *6 2.0 1.0 3.0 875     13500                                                                               1.2  1 × 10.sup.11                           *7 2.0 1.0 5.0 875     16100                                                                               1.6  9 × 10.sup.10                            8 2.0 3.0 0   950     8000 0.6  2 × 10.sup.12                            9 2.0 3.0 3.0 845     11300                                                                               0.8  6 × 10.sup.11                           10 2.0 3.0 5.0 845     12600                                                                               1.3  8 × 10.sup.10                           *11                                                                               4.0 0.5 0   950     12800                                                                               1.2  6 × 10.sup.11                           *12                                                                               4.0 0.5 3.0 845     15400                                                                               1.5  8 × 10.sup.10                           13 4.0 0.5 5.0 845     14700                                                                               1.6  8 × 10.sup.10                           14 4.0 1.0 0   950     10900                                                                               0.7  8 × 10.sup.11                           *15                                                                               4.0 1.0 3.0 810     13200                                                                               1.2  2 × 10.sup.11                           *16                                                                               4.0 1.0 5.0 810     12700                                                                               1.4  9 × 10.sup.10                           17 4.0 3.0 0   900      7700                                                                               0.5  1 × 10.sup.12                           18 4.0 3.0 3.0 810     10600                                                                               0.9  4 × 10.sup.11                           19 4.0 3.0 5.0 810      9800                                                                               1.1  3 × 10.sup.11                           __________________________________________________________________________      *indicates the scope of claims of the present invention.                 

As shown in Table 1, it is noted that anti-reduction characteristics of over 10 ¹⁰ Ω·cm in insulation resistance can be obtained only with the addition of CaO of 0.5 wt %. However, when CaO of 1.0 wt % is added, the dielectric constant is decreased, anti-reduction characteristics are increased, and sintering temperature is reduced. Meanwhile, the sintering temperature is lowered to 810° C. with the addition of PbO and CuO. In case that a large amount (over 3 mol %) of them is added, it is noted that the dielectric constant and insulation resistance are slightly reduced.

An MLC using such a dielectric ceramic composition will be described below.

Referring to FIG. 1, an MLC of the present invention comprises a dielectric layer 1, internal electrode 2, and outer electrode 3. For dielectric layer 1, composition [95PMN-5PT]-4(PbO)-5(CaO)-3(CuO) is selected among compositions having excellent dielectric characteristics of the aforementioned anti-reduction composition. The mixing method for the composition is the same as aforementioned.

The mixed powder is pulverized for two hours into a fine powder of below 1 μm in average particle diameter. Slurry is made in order to cast the fine powder as a thick film for MLC. The composition ratio of the slurry is dielectric powder: binder solution=64.0:36.0.

The binder solution is one of the polyvinylbutyral group and for the solvent media, a solvent system is used in which toluene and ethanol are mixed in a ratio of 15 wt %:85 wt %, respectively. Here, for the plasticizer, di-n-butyl phthalate of approximately 0.2 wt % is added. The volume ratio of the dielectric powder of the slurry is about 55 vol %. The slurry is used to make a 30 μm-thick sheet on a PE film by the doctor-blade method.

For the starting materials of paste for manufacturing copper internal electrode 2, CuO powder of 1 μm in diameter is used. For outer electrode 3, CuO powder of coarse particles having diameters of 3-5μm is used. As the organic binder solution, Vehicle #424 (Electro-Science Lab., U.S.A) is used. The composition ratio of the CuO paste for the internal electrode is CuO powder: binder solution=67.0:33.0. Here, the final viscosity of the paste is controlled with turpentine oil.

Internal electrode 2 is printed using an MLC pattern for 1206 size (EIA STD size; L=0.12 inch, w=0.06 inch) which is a standard size, with a stainless 400-mesh screen. The printed sheet is dried for four hours at 80° C. and then stacked to form six active dielectric layers. The result is isostatic-pressed under a pressure of 4,000 psi, and cut into chips with an automatic dicing saw. These unsintered green chips are put into an oven to burn out the organic binder therefrom. The organic binder burn-out and presintering temperatures are illustrated in FIG. 2.

The present invention is characterized in having a presintering process during firing. With such a presintering, first, the unsintered chips have adequate mechanical strength in order to perform an edge rounding or harperizing process for attaching the CuO paste used for outer electrodes prior to the main sintering. Secondly, the presintering prevents damage or fine cracks due to external impacts. Thirdly, carbon residues of the organic binder or CaCO₃ can be removed as completely as possible, preventing the insulation resistance from being reduced due to reduction sintering.

During edge rounding of the chips prior to sintering, productivity can be drastically improved as the rounding time can be reduced to 10-20 minutes from the conventionally required 2-6 hours because of ceramic of the presintering having a low strength. In addition, the chips are polished in a soft state, nearly eliminating possible chipping of the edges as compared with a conventional method in which polishing is done on the hardened chips after sintering, thus improving chip yield.

The presintered chips with their edges polished are ultrasonically washed, and CuO paste is attached thereto in order to form outer electrode 3. This CuO paste for the outer electrode is structured to be slightly porous in order to minimize the stress to the ceramic main body during sintering. For adhesion to the ceramic main body, CuO powder of 3-5 μm is used which is more coarse than the CuO paste for the internal electrodes. A small amount (2.5 wt %) of dielectric powder is added to match the dielectric ceramic during sintering. The composition ratio of the CuO paste for outer electrode of the present invention is CuO powder:binder solution:dielectric powder=72.0:25.5:2.5.

In the CuO paste compositions for internal and outer electrodes of the present invention, the CuO powder can be sufficiently sintered by a reactive sintering mechanism within the desired range of sintering temperature according to the present invention. For this reason, additives such as glass frits to aid the sintering process are not required.

The presintered chips with the outer electrode attached thereto are sintered in a furnace of reduction atmosphere. The mounting structure of the specimens is as shown in FIG. 3. These specimens are mounted in a mullite setter with application of calcination powder having the same composition as that of the specimen.

Referring to FIG. 4, a reduction firing furnace system of the present invention controls oxygen partial pressure at 800° C. This control temperature was determined by a stabilized zirconia oxygen sensor in which yttria of 6 mol % was added, using a miniaturized cell furnace and 800° C. was found to allow optimal controlability. This is because the main composition of the present invention is PbO and thus the function of the sensor is deteriorated due to the reaction of the Pt electrode attached to the oxygen sensor with the slight amount of oxygen PbO evaporated during sintering, to disable precise control of oxygen partial pressure. Thus oxygen partial pressure control of the present invention contributes to the elongation of the life span of the oxygen sensor.

The gas mixed in the miniaturized cell fuace is supplied to the main firing furnace. In the present invention, the mixed gas used for oxygen partial pressure control is H₂ --N₂ --H₂ O. Here, Ni-chrome wire is wound around the connection gases to maintain the temperature thereof above 100° C., to prevent oxygen partial pressure variation due to condensation of the pipes caused by cooling when the oxygen partial pressure controlled mixed gas passes to the sintering furnace. A sintering process used in the present invention is shown in FIG. 5.

The following table 2 shows the dielectric characteristics of the MLC specimen having a copper internal electrode sintered by the firing process of the present invention.

                                      TABLE 2                                      __________________________________________________________________________     Dielectric Properties having MLC of Copper Internal Electrode                                           Dielectric Properties                                                               Dielectric                                                                          Insulation                                  Additives (mol %)                                                                               Sintering                                                                              Dielectric                                                                          Loss Resistance                                  Example                                                                             x(PbO)                                                                             y(CaO)                                                                             z(CuO)                                                                             temperature (° C.)                                                              constant                                                                            (%)  (Ω · cm)                     __________________________________________________________________________     1    2.0 0.5 3.0 900     13500                                                                               2.6  7 × 10.sup.9                          2    2.0 0.5 5.0 900     12900                                                                               3.1  2 × 10.sup.9                          3    2.0 1.0 3.0 875     13600                                                                               2.8  .sup. 1 × 10.sup.10                   4    2.0 1.0 5.0 875     14200                                                                               2.9  8 × 10.sup.9                          5    4.0 0.5 3.0 845     13400                                                                               3.3  6 × 10.sup.9                          6    4.0 0.5 5.0 845     15200                                                                               2.7  8 × 10.sup.9                          7    4.0 1.0 3.0 810     13800                                                                               2.2  4 × 10.sup.9                          8    4.0 1.0 5.0 810     12100                                                                               2.4  1 × 10.sup.9                          __________________________________________________________________________

As shown in table 2, it is noted that the dielectric properties of the reduction sintered MLC is lower than those of the single-plate disk capacitor of table 1. This is because the CuO diffused to the dielectric layer in the soaking temperature range is partially reduced during reduction, thereby lowering the overall dielectric insulation resistance. Therefore, after internal electrode reduction, the present invention is reoxidated at in for two hours around 400° C. which is a low temperature for cooling, for enhancing insulation characteristics. The dielectric properties of the copper internal electrode MLC manufactured by the abovementioned method will be shown in the following table 3.

                                      TABLE 3                                      __________________________________________________________________________     Dielectric Properties of Reoxidated Copper Internal Electrode MLC                                       Dielectric Properties                                                               Dielectric                                                                          Insulation                                  Additives (mol %)                                                                               Sintering                                                                              Dielectric                                                                          Loss resistance                                  Example                                                                             x(PbO)                                                                             y(CaO)                                                                             z(CuO)                                                                             temperature (° C.)                                                              constant                                                                            (%)  (Ω · cm)                     __________________________________________________________________________     1    2.0 0.5 3.0 900     12400                                                                               1.9  3 × 10.sup.10                         2    2.0 0.5 5.0 900     12000                                                                               2.9  1 × 10.sup.10                         3    2.0 1.0 3.0 875     13100                                                                               2.2  4 × 10.sup.11                         4    2.0 1.0 5.0 875     15200                                                                               2.2  2 × 10.sup.11                         5    4.0 0.5 3.0 845     12900                                                                               1.7  8 × 10.sup.10                         6    4.0 0.5 5.0 845     14500                                                                               2.0  8 × 10.sup.10                         7    4.0 1.0 3.0 810     13300                                                                               1.8  5 × 10.sup.11                         8    4.0 1.0 5.0 810     12200                                                                               1.9  2 × 10.sup.11                         __________________________________________________________________________

As described above, the characteristics of the present invention are first, enabling sintering to be performed at a low temperature of up to 810° C., secondly, having a dielectric constant of over 12,000, a dielectric loss of below 3%, and an insulation resistance of 10⁹ Ω·cm, thirdly, using copper, a low-cost base metal, as the material for the internal and outer electrodes to thereby allow simultaneous firing at a reduction atmosphere together with the dielectric composition, and fourthly, enhancing insulation resistance through reoxidation during the cooling process of the reduction atmosphere firing process. 

What is claimed is:
 1. A method of manufacturing a multi-layered ceramic capacitor comprising the steps of:forming a slurry of a fine powder of a Pb-based dielectric ceramic composition; forming the slurry into a sheet form and printing a pattern of a copper internal electrode; drying and stacking said sheet on other sheets and cutting the sheets to thereby manufacture nonsintered chips; burning out organic binders of the nonsintered chips at temperatures of 280° C. to 430° C.; presintering the chips in air at temperatures of 700° to 900° C.; rounding edges of the presintered chips; forming copper outer electrodes on the edge-rounded presintered chips; sintering the presintered chips in air at 810° C. to 900° C.; reducing the internal electrode and outer electrode in a reduced air at a temperature of 800° C.; and performing reoxidation in air at a temperature of 400° C. during cooling of a firing.
 2. A method of manufacturing a multi-layered ceramic capacitor as claimed in claim 1, wherein the composition ratio of the slurry is dielectric powder: binder solution=64.0:36.0 in the step of forming slurry of the fine powder.
 3. A method of manufacturing a multi-layered ceramic capacitor as claimed in claim 1, wherein the composition ratio of the copper paste for the internal electrode is CuO powder:binder solution=67.0:33.0 in the step of printing the pattern of the copper internal electrode.
 4. A method of manufacturing a multi-layered ceramic capacitor as claimed in claim 1, wherein the composition ratio of the copper paste for outer electrode is CuO powder:binder:solution:dielectric powder=72.0:25.0:2.5 in the step of forming the copper outer electrode.
 5. A method of manufacturing a multi-layered ceramic capacitor as claimed in claim 1, wherein the sintering temperature is below 900° C. and the reoxidation temperature is 400° C.
 6. A method of manufacturing a multi-layered ceramic capacitor of claim 1, wherein the Pb-based dielectric ceramic composition comprises [95PMN-5PT]-x(PbO)-y(CaO)-z(CU) having a high dielectric constant wherein x=2 to 4 mol %, y-0.5 to 3.0 mol % and z=3 to 5 mol %. 